1. Field of the Invention
This invention relates generally to oil field tools, and more particularly to acoustic data telemetry devices for transmitting data from a downhole location to the surface.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth""s formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices in the BHA measure certain downhole operating parameters associated with the drill string and the wellbore. Such devices typically include sensors for measuring downhole temperature, pressure, tool azimuth, tool inclination, drill bit rotation, weight on bit, drilling fluid flow rate, etc. Additional downhole instruments, known as measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) and logging-while-drilling (xe2x80x9cLWDxe2x80x9d) devices in the BHA provide measurements to determine the formation properties and formation fluid conditions during the drilling operations. The MWD or LWD devices usually include resistivity, acoustic and nuclear devices for providing information about the formation surrounding the borehole.
The trend in the oil and gas industry is to use a greater number of sensors and more complex devices, which generate large amounts of measurements and thus the corresponding data. Due to the copious amounts of downhole measurements, the data is typically processed. downhole to a great extent. Some of the processed data must be telemetered to the surface for the operator and/or a surface control unit or processor device to control the drilling operations, which may include altering drilling direction and/or drilling parameters such as weight on bit, drilling fluid pump rate, and drill bit rotational speed. Mud-pulse telemetry is most commonly used for transmitting downhole data to the surface during drilling of the borehole. However, such systems are capable of transmitting only a few (1-4) bits of information per second. Due to such a low transmission rate, the trend in the industry has been to attempt to process greater amounts of data downhole and transmit only selected computed results or xe2x80x9canswersxe2x80x9d uphole for controlling the drilling operations. Still, the data required to be transmitted far exceeds the current mud-pulse and other telemetry systems.
Although the quality and type of the information transmitted uphole has greatly improved since the use of microprocessors downhole, the current systems do not provide telemetry systems, which are accurate and dependable at low frequencies of around 100 Hz.
Acoustic telemetry systems have been proposed for higher data transmission rates. Piezoelectric materials such as ceramics began the trend. Ceramics, however require excessive power and are not very reliable in a harsh downhole environment. Magnetostrictive material is a more suitable material for downhole application. Magnetostrictive material is a material that changes shape (physical form) in the presence of a magnetic field and returns to its original shape when the magnetic field is removed. This property is known as magnetostriction.
Most ferromagnetic materials exhibit some measurable magnetostriction; however, considerable field magnitudes are required which make such materials impractical for downhole use. However, greater magnetostriction can be obtained by using certain specially formulated alloys. For example, iron alloys containing the rare earth elements Dysprosium, and Terbium placed under adequate mechanical bias can produce strains to about 2000 microstrain in a field of 2 KOE at room temperature. Certain specifically formulated alloys have been found to exhibit sufficient magnetostriction with reasonable power consumption for use in downhole telemetry applications. One such alloy is commercially available under the brand name Terfenol-D(copyright).
Certain downhole telemetry devices utilizing a magnetostrictive material are described in U.S. Pat. Nos. 5,568,448 to Tanigushi et al. and 5,675,325 to Taniguchi et al. These patents disclose the use of a magnetostrictive actuator mounted at an intermediate position in a drill pipe, wherein the drill pipe acts as a resonance tube body. An excitation current applied at a predetermined frequency to coils surrounding the magnetostrictive material of the actuator causes the drill pipe to deform. The deformation creates an acoustic or ultrasonic wave that propagates through the drill pipe. The propagating wave signals are received by a receiver disposed uphole of the actuator and processed at the surface.
The above noted patents disclose that transmission efficiency of the generated acoustic waves is best at high frequencies (generally above 400 hz). The wave transmission, however drops to below acceptable levels at low frequencies (generally below 400 hz). The acoustic telemetry system according to the above noted patents requires precise placement of the actuator and unique xe2x80x9ctuningxe2x80x9d of the drill pipe section with the magnetostrictive device in order to achieve the most efficient transmission, even at high frequencies.
The precise placement requirements and low efficiency is due to the fact that such systems deform the drill pipe in order to induce the acoustic wave. In such systems, the magnetostrictive material works against the stiffness of the drill pipe in order to deform the pipe. Another drawback is that the deformation tends to be impeded by forces perpendicular (xe2x80x9cnormalxe2x80x9d or xe2x80x9corthogonalxe2x80x9d) to the longitudinal drill pipe axis. In downhole applications, extreme forces perpendicular to the longitudinal drill pipe axis are created by the pressure of the drilling fluid (xe2x80x9cmudxe2x80x9d) flowing through the inside of the drill pipe and by formation fluid pressure exerted on the outside of the drill pipe. Although the pressure differential across the drill pipe surface (wall) approaches zero with proper fluid pressure control, compressive force on the drill pipe wall remains. Deformation of the drill pipe in a direction perpendicular to the longitudinal axis is impeded, because the compressive force caused by the fluid pressure increases the stiffness of the drill pipe.
The present invention addresses one or more of the deficiencies of the above-noted acoustic telemetry systems, and provides a telemetry system wherein a magnetostrictive actuator deflects (moves) a tube body along a longitudinal direction thereof relative to a reaction mass. The reaction mass is separated from the tube body through which the transmission of the acoustic wave generated by the magnetostrictive actuator is desired. The mass of the reaction mass is substantially greater than the mass of the tube body, which allows the tube body to move relative to the reaction mass, thereby allowing transmission of the generated acoustic waves, even at a relatively low frequencies.
In one embodiment, the present invention includes, an elongated member (also referred to herein as the xe2x80x9csignal conducting massxe2x80x9d), such as a drill pipe, that is a capable of conducting acoustic waves therethrough, a reaction mass and an acoustic actuator coupled to the elongated member and the reaction mass. The acoustic actuator generates axial force between the elongated member and the reaction mass at a predetermined frequency. The effective mass of the reaction mass is greater than the mass of the elongated member by an amount that is sufficient to cause a substantial portion of the axial force generated by the acoustic actuator to be applied to the elongated member. The axial force applied to the elongated member produces an acoustic wave at the predetermined frequency, which is transmitted through the elongated member.
In one embodiment of the present invention, the acoustic actuator is disposed in a drill string wherein the portion of the drill string uphole of the acoustic mass forms a movable elongated member and the portion of the drill string below or downhole of the acoustic actuator forms the reaction mass. During drilling of a wellbore, the drill string portion below the acoustic actuator is substantially immovable since the portion""s axial movement is stopped by the wellbore bottom. Thus, the lower portion of the drill string acts as a reaction mass whose effective mass is many times greater than the drill string upper portion. Since the drill string upper portion is movable relative to the reaction mass, a substantial portion of the axial force generated by the acoustic actuator is transmitted into the drill string upper portion.
In an alternative embodiment of the present invention, the reaction mass may be a weight disposed within a drill string or it may be obtained by anchoring in the borehole a drill string section that is positioned below the acoustic actuator. The acoustic actuator includes a magnetostrictive element disposed between the signal conducting or transmitting mass and the reaction mass. A controller energizes coils disposed around the magnetostrictive element at a predetermined frequency, which causes the magnetostrictive material to simultaneously apply axial force to the signal conducting mass and the reaction mass. The effective mass of the reaction mass being significantly greater than the signal conducting mass causes a substantial portion of the axial force generated by the acoustic actuator to be applied to the signal conducting mass.
The present invention provides a magnetostrictive apparatus and a method for efficiently and effectively transmitting signals from a downhole location through a pipe such as a drill pipe or production pipe at low frequencies with high efficiencies. The apparatus and methods of the present invention may be utilized as a telemetry system in the drill string to transmit signals and data during drilling of wellbores or as a part of completion well and production well telemetry systems.
The present invention includes a signal conducting mass such as a metallic pipe, a reaction mass at least one actuator in a coupling arrangement with the signal conducting mass and the reaction mass for inducing an acoustic wave representative of a parameter of interest, wherein the mass of the reaction mass is greater than the signal conducting mass such that substantially all of the acoustic wave is transferred to the signal conducting mass. An acoustic wave receiver disposed in the signal conducting mass receives the acoustic wave and converts such acoustic wave to a signal indicative of the one parameter of interest. A processor processes the second signal from the acoustic wave receiver and determines the parameter of interest. The actuator includes a magnetostrictive member that exerts force on the signal conducting mass and the reaction mass at a predetermined frequency to induce the acoustic wave in the signal conducting mass. In one embodiment of the present invention, a section of the drill string below or downhole of the actuator is utilized as the reaction mass while the section of the drill string above or uphole of the actuator is utilized as the signal conducting mass. In an alternative embodiment, a portion of the pipe is firmly anchored in the wellbore and a section on one side of the anchor is utilized as the signal conducting mass while the earth is used as the reaction mass.